Young will report to Triple-A Las Vegas. He will give the Mets a capable pinch runner for the bench once rosters expand in September.
"He's a guy we know pretty well," assistant GM John Ricco said. "He brings a different dimension to the game certainly with his speed. He's a guy we know well, who will fit into the clubhouse, and provides a different dimension."
Young, 30, hit .169 with five RBIs and three steals in 85 plate appearances to begin the season with Atlanta. He has been playing with Triple-A Gwinnett since mid-June.
Young hit a combined .242 and had 68 steals with the Mets in 191 games between 2013 and '14 after being acquired from the Colorado Rockies for Collin McHugh. He was non-tendered by the Mets in December.
"It's a dimension we don't have," manager Terry Collins said. "It would be very important to have a guy like that. He knew his role and he got ready to play. Every day he came with a smile on his face and did his job."